Last edited 26 May 2024

Should Part O be extended to existing buildings?

1 Background to Part O for new buildings
2 Existing built environment context
3 Proposing part O for existing buildings
4 Green Infrastructure Framework (GIF)
5 Overheating guidance and methodologies
- 5.1 Part O simplified method of compliance
- 5.2 Part O dynamic modelling method of compliance
6 Good Homes Alliance new and existing homes guidance
7 Related articles on Designing Buildings

[edit] Background to Part O for new buildings

The approved Document O came into full effect in June of 2023, as the transitional arrangements ended and is now applicable to all new buildings. In the build up to and following its publication a number of further guidance documents were also published some of these also covered guidance for existing buildings.

The Requirements of O1 Overheating mitigation in new buildings: 'Reasonable provision must be made in respect of a dwelling, institution or any other building containing one or more rooms for residential purposes, other than a room in a hotel (“residences”) to

(a) limit unwanted solar gains in summer
(b) provide an adequate means to remove heat from the indoor environment.

In meeting these obligations for new buildings

(a) account must be taken of the safety of any occupant, and their reasonable enjoyment of the residence
(b) mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it.

The aims of the requirement being to protect the health and welfare of occupants of a building by reducing the occurrence of high indoor temperatures, with the focus being on new buildings only.

In April 2024 the Environmental Audit Committee highlighted its expectations for the Government to provide more detail on its plans to adapt the UK to climate change. Following the report Heat Resilience and Sustainable Cooling, the committee branded Government answers on adapting to heatwaves as a “missed opportunity”.

For more information about Part O

[edit] Existing built environment context

The Environmental Audit Committee (EAC) highlights that four out of five homes set to exist by 2050 have already been built, as the UK has some of the oldest housing stock in the world.

It found that while the UK population spends on average 90% of its time indoors, the country’s housing stock is not designed to cope with excessive heat, and millions of UK homes experience summertime overheating. The Committee cited findings that heat-related deaths could rise to 10,000 every year without concerted actions to adapt to the warming climate.

The Committee recommended launching an ambitious and comprehensive national retrofit programme to adapt the UK’s houses for the demands of net zero. It recommended this be delivered via local authorities, be underpinned by long-term funding and prioritise vulnerable households.

[edit] Proposing part O for existing buildings

The Environmental Audit Committee (EAC) also recommended that standards covering overheating in new buildings, the Part O of the Building Regulations 2010, should be extended to cover not just material changes of use but also refurbishment of existing properties.

In its report, it recommended that Ministers set out whether the Government would pursue this, or if not, explain how the UK would achieve its commitments on global cooling. Such as the UN Global Cooling Pledge, an initiative stemming from the 2023 United Nations Climate Change Conference (COP28) and to which the UK is a signatory. The initiative aims to raise ambition and international cooperation through collective global targets to reduce cooling related emissions by 68% from today by 2050, significantly increase access to sustainable cooling by 2030, and increase the global average efficiency of new air conditioners by 50%.

The Government has as yet not said whether it plans to extend the part O regulations for existing buildings. The Ministers have said they plan to use a call for evidence currently running to understand any potential problems with the Part O regulations, and to determine future updates.

The consultation seeking evidence on previous changes to Part O (overheating) has now closed.

[edit] Green Infrastructure Framework (GIF)

Whilst part O currently does not significantly include considerations to localised tree shading, the establishment of the Green Infrastructure Framework (GIF) by Natural England was welcomed. However the Climate Change Committee also found that there was “no requirement for local authorities to protect or provide green space”, and that funding commitments were required to maintain parks and green spaces.

The Green Infrastructure Framework is a commitment in the Government’s 25 Year Environment Plan. It supports the greening of our towns and cities and connections with the surrounding landscape as part of the Nature Recovery Network. Networks of green and blue spaces and other natural features can bring big benefits for nature and climate, health and prosperity. At present access to green and blue spaces varies considerably across the country, and there are opportunities for these important assets to be better managed for the environment and to deliver a wider range of multifunctional benefits.

To find out more about the Green Infrastructure Framework visit here.

[edit] Overheating guidance and methodologies

[edit] Part O simplified method of compliance

The simplified method for limiting solar gains and providing a means of removing excess heat, is the first compliance method for new buildings which includes.

Categorisation of residential buildings made according to their location and the inclusion or not of cross-ventilation, this is to limit unwanted summer solar gain and remove excess heat. Limiting solar gains needs to be achieved in this model by setting maximum glazed areas of the buildings, and of the most glazed room as well as providing shading for high-risk locations, the acceptable strategies for doing this are listed in the regulations as:

Fixed shading devices, comprising any of the following: Shutters, external blinds, overhangs, awnings,
Glazing design, involving any of the following solutions: Size, orientation, g-value, depth of the window reveal.
Building design – for example, the placement of balconies.
Shading provided by adjacent permanent buildings, structures or landscaping.

NOTE: Although internal blinds and curtains as well as foliage, such as tree cover, can provide some reduction in solar gains, they should not be taken into account when considering whether the requirement has been met.

The removal of excess heat is to be achieved through cross ventilation approaches, which rely on the correct calculation of minimum free areas for windows and vents which is given, this guidance varies depending on the level of risk for assessed though the building's location The regulation lists the following means for opening windows (the effectiveness of this method is improved by cross-ventilation).

NOTE: Passive means must be demonstrated as far is reasonably possible before mechanical cooling, approaches must comply with the other relevant regulations, such as Document F Considering opening lights and purge ventilation.

[edit] Part O dynamic modelling method of compliance

To demonstrate compliance using the dynamic thermal modelling method, for new buildings all of the following guidance should be followed.

CIBSE’s TM59 methodology for predicting overheating risk.
The limits on the use of CIBSE’s TM59 methodology set out in paragraphs 2.5 and 2.6.
The acceptable strategies for reducing overheating risk in paragraphs 2.7 to 2.11.
Find out more at: https://www.gov.uk/government/publications/overheating-approved-document-o

[edit] Good Homes Alliance new and existing homes guidance

The Good Homes Alliance published an overheating guidance and risk score sheet tool for new homes in July 2019 followed by the same for retrofit and existing homes in March 2022. These are tools for early stage assessment not detailed assessment, sitting between existing high-level guidance, more detailed calculations and modelling tools, identifying key factors contributing to overheating risks, and possible mitigation measures.

For more information visit: https://goodhomes.org.uk/overheating-in-new-homes. an d https://kb.goodhomes.org.uk/tool/overheating-retrofit/

[edit] Zero Carbon Hub evidence base definitions

Highlights that evidence-based ‘overheating’ thresholds related to different sectors have been developed on the basis of different environmental variables, quite often by researchers from different disciplines. As a result, they are commonly expressed in different metrics and are therefore not directly comparable with each other. Indoor health-related thresholds are less well defined in comparison to thermal comfort-based thresholds, despite the well-characterised epidemiological relationships between outdoor ambient temperature and heat-related morbidity and mortality. This is partly due to the methodological complexity of linking indoor environments with health outcomes. It defines more clearly the term, overheating in its context and indicates that future research should aim to establish an integrated approach towards defining overheating thresholds that cuts across comfort, wellbeing and health impacts.

For more information visit:

https://www.zerocarbonhub.org/sites/default/files/resources/reports/ZCH-OverheatingEvidenceReview-Definitions.pdf

[edit] Thermal comfort ISO 13792:2005

Overheating in buildings is most commonly defined in terms of thermal comfort, but it can also be related to and assessed in terms of health, well-being and productivity. The International Standard defines thermal comfort as ‘that condition of mind that expresses satisfaction with the thermal environment’ (ISO 2005). The approaches taken to assess overheating in buildings differs, depending on the type and use of the building.

For more information visit: https://www.iso.org/standard/37059.html

[edit] Temperature limits for buildings CIBSE guide A 2006

A general temperature limit for buildings (CIBSE Guide A-2006) is where an indoor comfort criteria is given as 25ºC, and 28ºC is given as the maximum allowable temperature, for no more than 1% of the occupied hours, when using a design summer year weather file for a climate modelling approach.

For more information visit: https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I79JAAS

[edit] Temperature limits for buildings CIBSE guide A update 2015

In 2015 the CIBSE Guide A was updated to incorporate considerations for adaptive thermal comfort. Here the Indoor Comfort Temperature (operative temperature in °C) is 0.33Trm + 18.8 and the Maximum Temperature (°C) or Comfort Temperature is +3. Using design summer years the Operative temperature can only surpass the max temperature for no more than 3% of the total occupied hours.

For more information visit: https://www.cibse.org/knowledge/knowledge-items/detail?id=a0q20000008I79JAAS

[edit] Weather data reference files

Design Summer Year (DSY) is a weather data reference file, that represents an averaged year of weather, as if it were single and continuous, rather than monthly. Adapted probabilistic DSYs to better describe overheating events, frequency and severity, the simple approach is though less able to account for extreme temperature events or incident radiation. The Test Reference Year (TRY) is similar but one that models or represents a typical year in a certain location. A greater number of reference points, makes it more complex but more accurate (depending on the years selected). TRY files have been updated at various points in time and are available for 14 locations across the UK. More recently with the onset of climate change, many organisations start to use TRYs for different future climate scenarios.

Future Climate weather files; DSY's and TRY's incorporate different climate change scenarios (UK climate projections) with different percentiles of likely hood for three different time periods: 2020s (2011-2040), 2050s (2041-2070) 2080s (2071-2100). The projections are based on different emission scenarios (Low - High) relating to climate mitigation efforts, which directly relate to the representative concentration pathways (RCPs) used show likely global mean temps (RCP2.6, 4.5, 6.0 and 8.5). Percentiles describe different probabilistic assumptions within the scenarios, for example, a 90th Percentile high emissions scenario means a 10% chance that temperatures will fall above the given threshold for a particular time period (worst case scenario), whilst 10th percentile low emissions scenario is a better case.

For more information visit: https://www.cibse.org/weatherdatasets

[edit] Over heating in schools BB101

Schools introduced the use of three overheating criteria in BB101. The duration of overheating, when indoor temperature may rise above 28ºC but for no more than 120 hours per year. The severity of overheating; where the averaged difference between the internal temperature and the external temperature (Tint – Text) remains less than 5ºC per day. As well as an upper temperature limit of 32ºC, that should not be exceeded. The specification guidance includes for the construction of new school buildings, as well as the refurbishment of existing schools.

For more information visit: https://www.gov.uk/government/publications/building-bulletin-101-ventilation-for-school-buildings

[edit] Adaptive comfort theory

An adaptive comfort model take these three criteria, but also considers how an individual’s thermal expectations or preferences are impacted by their recent experiences of (outdoor) temperature, as well as a range of contextual factors. In practical terms this allows the human physiology to naturally adapt to extended periods in warmer (or cooler) environments.

For more information visit: https://www.cibse.org/getattachment/Networks/Regions/South-Wales/South-Wales-Past-Presentations/TM52-The-limits-of-thermal-comfort-Cardiff.pdf.aspx

[edit] CIBSE TM52

The CIBSE TM52 (2013) guidance, for non-domestic buildings, considers adaptive comfort criteria, introducing a ‘running mean temperature’ (Trm) which is a rolling average of the outdoor air temperature, weighted according to time in the past and an operative temperature (Top) which is a combination of the air temperature and the mean radiant temperature. In this model the duration of overheating or operative temperature (°C or (He) ΔT ≥ 1) does not exceed 3% of the occupied hours. The severity of overheating uses a weighted exceedance (We), less or equal to 6 and finally where the maximum operative temperature does not exceed a temperature difference (ΔT) of 4ºC. This guidance was then adapted for schools to also consider the impact of cold drafts, with an additional requirement to mix ventilated air with room air to prevent this. It contains a broad range of guidance including various explanations and the use of predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) in the heat balance model of comfort.